Liquid crystal displays have a structure in which a liquid crystal layer is sandwiched between a pair of substrates, and can express light and dark utilizing the electro-optical response caused by external electric field. They can also display colors by using color filters comprising pixels having color selectivity.
In liquid crystal displays, black matrix is used as a light shading film for shading the regions at which prevention of light transmittance is required, such as the regions between the pixels, and the regions carrying driving circuits. As the materials forming the light shading film, metallic materials having large extinction factors, such as chromium, nickel and aluminum, are used.
The light shading films made of these metals are usually formed by vapor deposition, sputtering or vacuum deposition, and the patterning of the formed light shade films is carried out by photolithography. In a typical method, the black matrix is formed by coating the metal thin film formed by the above-mentioned method with a photoresist, drying the coated photoresist, irradiating the dried photoresist with ultraviolet light through a photomask to form a resist pattern, etching the metal thin film and by peeling off the photoresist. The thus obtained black matrix made of a metal thin film has a film thickness of about 0.2 μm, and an optical density (OD value) of not less than 4.0. In color filters using the black matrix made of a metal thin film, the height of the surface steps after forming the red, green and blue pixels is as small as not more than 0.2 μm, so that the color filters have an advantage that formation of an over-coat for decreasing the height of the steps is not necessary. However, the production process of the black matrix made of a metal thin film is complicated so that the production cost is high. As a result, there is a problem in that the cost of the color filters is high accordingly. Further, with the liquid crystal displays having a black matrix made of a metal thin film, since the reflectance at the surface of the metal thin film is high, the reflected light is strong, so that the display quality is drastically degraded, which is problematic. To decrease the reflectance, a method has been proposed in which a metal oxide film is formed between the metal thin film and the substrate to form a bilayered or trilayered structure. However, even with this method, the problem of the production cost cannot be overcome.
On the other hand, as light shading agents, carbon black, titan black and the like have been used. The black matrix using the light shading agent such as carbon black or titan black is generally prepared by the following process:
First, a black composition comprising a light shading agent dispersed in a resin solution is prepared. Then the black composition is applied on a substrate, and patterning of the black composition is conducted by photolithography in the same manner as described above. The thus obtained black matrix has advantageous features that the production cost is low because the film is formed by the paste-coating method and the reflectance can be made lower than that of the metal light shading film. The “paste-coating method” herein means the method in which the composition (paste) is applied on a substrate and the composition is then patterned by photolithography. However, the OD value per a unit film thickness thereof is much lower than those made of the metal thin films described above, so that it is necessary to increase the concentration of the light shading agent contained in the film, or to increase the thickness of the film for securing the sufficient light shading property. However, increasing the concentration of the light shading agent causes a problem in that the adhesiveness of the film with the substrate is decreased because the polymer content is decreased. On the other hand, if the thickness of the film is increased, the height of the surface steps after forming the red, green and blue pixels is made large, so that formation of an over-coat on the red, green and blue pixels is necessary in order to decrease the height of the surface steps.
As a black matrix, resin black matrix comprising carbon black dispersed in a non-photosensitive polyimide resin is known (Patent Literature 1). This resin black matrix has an improved dispersion stability by using a carbon black having a defined amount of functional groups at its surface, and has an OD value per 1 μm of film thickness of 3.1 to 3.4.
A resin black matrix comprising titan black and a resin is also known (see Patent Literature 2). The OD value of this black matrix is 3.0 at a film thickness of 0.9 μm. Further, a resin black matrix comprising titanium nitride oxide and a resin is also known (see Patent Literature 3). This resin black matrix attains increase in the OD value by employing a titanium nitride oxide whose R1 defined below is not less than 0.24. The maximum OD value of this resin black matrix per 1 μm of film thickness is 3.72.R1=I3/{I3+1.8(I1+1.8I2)}wherein
I1 represents the maximum diffraction intensity when the angle of diffraction 2θ is 25° to 26°;
I2 represents the maximum diffraction intensity when the angle of diffraction 2θ is 27° to 28°; and I3 represents the maximum diffraction intensity when the angle of diffraction 2θ is 36° to 38°.
The term “2θ” herein means the angle of diffraction of the titanium nitride oxide determined by using CuKα line as the X-ray source. A resin black matrix comprising titanic acid and a resin is known (Patent Literature 4). This resin black matrix has an increased OD value by using a titanium nitride oxide having a psychometric lightness L defined by Hunter's color difference equation of not more than 12.0 (The maximum OD value per 1 μm of film thickness is 4.0). A black radiosensitive resin composition comprising titan black is known (see Patent Literature 5). This resin black matrix has an OD value of 3.0 at a film thickness of 1.1 μm. A black pigment composition comprising a macromolecular compound and titan black is known (see Patent Literature 6). This resin black matrix has an OD value of 3.5 at a film thickness of 1.5 μm.
With any of the above-described known black matrices, the OD value per 1 μm of film thickness is not so high because the light shading property of the light shading agent used is not so high. Although a black matrix with which the OD value per 1 μm of film thickness is 4.0 is described as an example of the above-described resin black matrix comprising titanium nitride oxide and a resin, the concentration of the light shading agent in this resin black matrix is high (the weight ratio of the light shading agent to the resin is 70:30) in order to attain a high OD value. When the concentration of the light shading agent is high, the adhesion between the resin black matrix and the glass is not sufficient. There is also a problem in that a pattern of the resin black matrix cannot be formed in cases where the width of the pattern is small. This problem is even more serious in case of black radiosensitive resin compositions. That is, in addition to the problem in the adhesion, there is also a problem in that the edge portions have tapered shape or inverse tapered shape, and the vertical edge shape desired for the resin black matrix cannot be obtained.
In fact, with the color filters using a resin black matrix, the problem in that the adhesion of the resin black matrix is poorer than in the case of using metal thin film black matrix, that is, the problem in that the black matrix is peeled at the sealed portions, and the problem in that a narrow pattern having a width of not more than 10 μm cannot be formed have actually occurred.    Patent Literature 1: Japanese Patent No. 3196638 (pages 1 and 9-11, and Table 1)    Patent Literature 2: Japanese Laid-open Patent Application (Kokai) No. 2000-66018 (pages 2, 7-8)    Patent Literature 3: Japanese Laid-open Patent Application (Kokai) No. 2000-143985 (pages 2 and 5-7)    Patent Literature 4: Japanese Laid-open Patent Application (Kokai) No. 2001-40292 (pages 2 and 5-7)    Patent Literature 5: Japanese Patent No. 3230800 (pages 1 and 11)    Patent Literature 6: Japanese Laid-open Patent Application (Kokai) No. 10-114836 (pages 2 and 9-10)